1. Field of the Invention
The present invention relates to semiconductor devices, and particularly to potential control of a dynamic random access memory.
2. Description of the Background Art
A personal computer and a workstation each include a memory for storing data. An example of the memory is a DRAM (Dynamic Random Access Memory) with a great capacity from and into which data can be read and written. The DRAM is used as a main memory in the personal computer, workstation and the like. In recent years, an embedded DRAM composed of a DRAM and a logic circuit such as arithmetic circuit which are mounted on the same chip has been employed in a digital still camera and a notebook personal computer.
A memory cell of the DRAM is constituted of one transistor and one capacitor. One memory cell stores 1 bit of information depending on whether or not charges are stored in the capacitor. Generally the supply voltage is reduced in order to decrease power consumption of the DRAM. However, just the reduction of the supply voltage results in decrease of the amount of charges stored in the memory cell. In order to avoid this decrease, capacitance of the capacitor may be increased. Increase of the capacitance is achieved by forming the capacitor as a stacked type having a three-dimensional structure so as to increase the area of the capacitor electrode, or forming a dielectric film of the capacitor of material having a high dielectric constant (high dielectric material) so as to increase relative dielectric constant.
The techniques of forming the capacitor having the three-dimensional structure or forming the dielectric film of high dielectric material both have been employed. However, in order to further decrease the power consumption by reducing the voltage, a capacitor of a more complicated three-dimensional structure or material with higher dielectric constant is requested. This complicates the manufacturing process, resulting in increased manufacturing cost.